On August 1, 2008, at just about 8 p.m., a massive serac cleaved from a glacier near the summit of K2, the world's second highest mountain, and barreled down a section of the Cesen climbing route called the Bottleneck. In an instant, one climber was dead, key safety lines were swept away, and 17 climbers were trapped above 27,000 feet with little chance of escape.

In the days ahead, the disaster on K2 would become one of the deadliest mountaineering incidents in history, leaving 11 victims in its wake. The tragedy would shake modern mountaineering to its core. And it would yield a hero, Pemba Gyalje Sherpa.

Pemba, 34, and three members of his Norit K2 team—leader Wilco van Rooijen, Marco Confortola, and Gerard McDonnell—reached the Bottleneck minutes after the serac fell. Rather than face a dangerous descent in total darkness, Pemba's three teammates decided to bivouac for the night. At 27,000 feet the temperatures would reach minus 40ºF. Pemba, a seven-time Everest veteran, knew the dangers of the death zone. He chose instead to descend the Bottleneck alone, without oxygen, picking his way down the 60-degree couloir guided by a single tattered safety line that had survived the avalanche. He reached Camp IV by 1 a.m. His teammates, he assumed, would be down at first light.

By daybreak on August 2, chaos reigned. More than a dozen climbers were missing or dead, and the weather had worsened considerably. Van Rooijen had staggered away from the team, desperate to get down by a different route, and soon became hopelessly lost. McDonnell had wandered back uphill, apparently confused. Frostbitten and delirious, Confortola had climbed partway down the Bottleneck, unable to remember how he'd done it. Just before he passed out from altitude sickness, a second avalanche swept toward him carrying McDonnell's mangled corpse.

With his team in shambles, Pemba had to act fast. He heard over the radio that Confortola had been spotted midway up the Bottleneck. "I thought, OK, if we are lucky, I can rescue Marco," Pemba says. So he began to climb, soloing through swirling snow up the couloir. "It was very scary, but I knew Marco was still alive," he says. "I could not turn back."

When Pemba reached Confortola some hours later, the Italian was in bad shape, unconscious and suffering from severe altitude sickness. Somehow Pemba managed to revive him with oxygen and guide him to the base of the Bottleneck. At that moment another slide roared from above, this time carrying the bloodied bodies of two Sherpas and two Korean climbers. A chunk of falling ice blasted Confortola in the back of the head. Dazed, the Italian began to slip. "I was falling," he told a reporter. "The avalanche would have taken me away. But Pemba grabbed me from behind. He was holding my neck. He saved my life."

By the time the pair made it to Camp IV, Pemba was shattered, collapsing into his tentfor a few hours' sleep. When he woke that evening, he got word that van Rooijen, the lost Norit K2 leader, was still alive. He had to go out again.

After a night alone in the open with no water and no ice ax, van Rooijen had been presumed dead. Then, unexpectedly, he called his wife on his satellite phone. Using the call data, the Norit K2 team fixed his location on the mountain's South Face, far from any known routes.

Armed with only rough coordinates, Pemba, along with another survivor, Cas van de Gevel, struck into terra incognita, picking across avalanche-prone terrain at night. After searching for hours, the pair decided to resume the next day. They finally found van Rooijen in the late afternoon by following the sound of his ringing cell phone. The three men staggered into Camp III well after dark, on August 3, exhausted but alive.

In the weeks after the tragedy, Pemba returned to his Kathmandu home, far from the horrors he'd just witnessed. You'd think that after such an experience, he would never want to climb again, soured forever. But Pemba has no such plans. He'll be back in the mountains, he says, by the time next season rolls around. Thank goodness. Climbing needs more heroes like him.